I. Field of the Invention
The present invention relates generally to electromagnetic communications, and more particularly, to a method and system for ensuring reception of a communications signal.
II. Description of the Related Art
Communication links utilize electromagnetic signals (EM), in the form of electromagnetic waves, to carry analog or digital electronic information from a first location to a second location. In doing so, a baseband signal, containing the information to be transmitted, is impressed on an oscillating signal to produce a modulated signal at the first location. The modulated signal is sent over the communications link to the second location. At the second location, the modulated signal is typically down-converted to a lower frequency, where the baseband signal can be recovered.
All EM signals can be sufficiently described in both the time domain and the frequency domain. FIG. 1A depicts a baseband signal 102 in the time domain that starts at time t.sub.0 and ends at a time t.sub.1. The baseband signal 102 can represent any number of real world occurrences. For example, baseband signal 102 could be the voltage output of a microphone for a given acoustical input. FIG. 1B illustrates spectrum 104, which is the frequency domain representation of baseband signal 102. Spectrum 104 depicts the relative amplitude of the sinusoidal components that when summed together with the correct relative phase will construct baseband signal 102 in the time domain. In other words, the spectrum 104 represents the relative amplitude and phase of the sine waves that constitute baseband signal 102 in the time domain.
Theoretically, a time-limited baseband signal (like baseband signal 102) has an infinite number of sinusoidal frequency components. That is, the "tail" of spectrum 104 will continue to infinity. However, the amplitude of the sinusoidal components in spectrum 104 decrease with increasing frequency. At some point, the higher frequency components can be ignored and filtered out. The highest frequency remaining defines the "frequency bandwidth" (B) of the spectrum 104. For example, if spectrum 104 corresponded to a human voice signal, the bandwidth (B) would be approximately 3.5 KHz. In other words, those sine waves beyond 3.5 KHz can be filtered out without noticeably affecting the quality of the reconstructed voice signal.
The signal with the simplest frequency domain representation is that of a single sine wave (or tone) at a given frequency f.sub.0. Sine wave 106 having a frequency f.sub.0, and its spectrum 108 are shown in FIGS. 1C, and 1D, respectively. Sinusoidal signals are one type of periodic signals (or repeating signals) that may also be referred to as "oscillating signals".
Amplitude modulation, a common modulation scheme, will be explored below to illustrate the effects of modulation. FIGS. 1E and 1F illustrate modulated (mod) signal 110 and its corresponding modulated spectrum 112. Modulated signal 110 is the result of amplitude modulating sine wave 106 with baseband signal 102. In the time domain, the amplitude of modulated signal 110 tracks the amplitude of the baseband signal 102, but maintains the frequency of sine wave 106. As such, sine wave 106 is often called the "carrier signal" for baseband signal 102, and its frequency is often called the "carrier frequency." In this application, information signals that are used to modulate a carrier signal may be referred to as "modulating baseband signals".
In the frequency domain, amplitude modulation causes spectrum 104 to be "up-converted" from "baseband" to the carrier frequency f.sub.0, and mirror imaged about the carrier frequency f.sub.0, resulting in modulated spectrum 112 (FIG. 1F). An effect of the mirror image is that it doubles the bandwidth of modulated spectrum 112 to 2B, when compared to that of modulated spectrum 104.
Modulated spectrum 112 (in FIG. 1F) is depicted as having substantially the same shape as that of modulated spectrum 104 (when the mirror image is considered). This is the case in this example for AM modulation, but in other specific types of modulations this may or may not be so as is known by those skilled in the art(s).
Modulated spectrum 112 is the frequency domain representation of what is sent over a wireless communications link during transmission from a first location to a second location when AM modulation is used. At the second location, the modulated spectrum 112 is down-converted back to "baseband" where the baseband signal 102 is reconstructed from the baseband spectrum 104. But in order to do so, the modulated spectrum 112 must arrive at the second location substantially unchanged.
During transmission over the wireless link, modulated spectrum 112 is susceptible to interference. This can occur because the receiver at the second location must be designed to accept and process signals in the range of (f.sub.0 -B) to (f.sub.0 +B). The receiver antenna accepts all signals within the stated frequency band regardless of their origin. As seen in FIG. 1G, if a second transmitter is transmitting a jamming signal 114 within the band of (f.sub.0 -B) to (f.sub.0 +B), the receiver will process the jamming signal 114 along with the intended modulated spectrum 112. (In this application a jamming signal is any unwanted signal regardless of origin that coexists in a band occupied by an intended modulated spectrum. The jamming signal need not be intended to jam.) If the power of jamming signal 114 is sufficiently large, then modulated spectrum 112 will be corrupted during receiver processing, and the intended information signal 102 will not be properly recovered.
Jamming margin defines the susceptibility that a modulated spectrum has to a jamming signal. Jamming margin is a measurement of the maximum jamming signal amplitude that a receiver can tolerate and still be able to reconstruct the intended baseband signal. For example, if a receiver can recover info signal 102 from spectrum 112 with a maximum jamming signal 114 that is 10 dB below the modulated spectrum 112, then the jamming margin is said to be -10 dBc (or dB from the carrier).
Jamming margin is heavily dependent on the type of modulation used. For example, amplitude modulation can have a typical jamming margin of approximately -6 dBc. Frequency modulation (FM) can have a jamming margin of approximately -3 dBc, and thus more resistant to jamming signals than AM because more powerful jamming signals can be tolerated.
The Federal Communications Commission (FCC) has set aside the band from 902 MHZ to 928 MHZ as an open frequency band for consumer products. This allows anyone to transmit signals within the 902-928 MHZ band for consumer applications without obtaining an operating licence, as long as the transmitted signal power is below a specified limit. Exemplary consumer applications would be wireless computer devices, cordless telephones, RF control devices (e.g. garage door openers), etc. As such, there is a potentially unlimited number of transmitters in this band that are transmitting unwanted jamming signals.
The 900-928 MHZ frequency band is only a single example of where jamming is a significant problem. Jamming problems are not limited to this band and can be a potential problem at any frequency.
What is needed is an improved method and system for ensuring the reception of a modulated signal in an environment with potentially multiple jamming signals.
What is also needed is a method and system for generating a modulated signal that is resistant to interference during transmission over a communications link.
What is further needed is a method and system for generating a modulated signal that has a higher inherent jamming margin than standard modulation schemes (e.g. AM, FM, PM, etc.), without substantially increasing system complexity and cost.